Digital images are often created or obtained in layers. For example, a foreground layer and a background layer, each of which is an image in its own right, may be composed together to form a combined image. When image layers are to be composed (a.k.a., composited) together, portions of one or more layers may be partially or fully transparent, so that layers underneath show through. The information that specifies transparency is often kept in an alpha channel. The alpha channel, sometimes referred to as a matte or a mask, specifies the transparency of pixels, whereas RGB (red green blue) and other channels specify the color of the pixels. Alpha values typically range from 0 (fully transparent) to 1 (fully opaque). Partial transparency is implemented by mixing colors.
Alpha compositing can be used to combine an image with a background to create the appearance of partial or full transparency. After rendering or importing image elements in separate passes, alpha compositing combines the images into a single final image. Compositing is often used to lay computer rendered text or other computer generated images on top of live video footage. An alpha channel matte for the computer generated image specifies the shape of the generated geometry, making it possible to distinguish between parts of the overall image where the generated geometry was drawn and other parts of the image which will contain the live footage.